Martian Ice Unearthed

Vast quantities of ice may lurk just beneath the arid surface of Mars, researchers report in three papers to appear in the 31 May issue of Science. The reports flesh out preliminary findings announced earlier this year and bolster suspicions that Mars is hiding large amounts of water.

For decades, planetary scientists have believed that dusty, dry Mars may once have been a wet place, and recent observations even suggest that torrential flooding carved some of the younger features on the face of the planet (ScienceNow, 4 December 2000). But researchers have wondered where all that water went. In March, preliminary results from NASA's Mars Odyssey orbiter suggested that much of the water was stashed away as ice lying just beneath the surface (ScienceNOW, 3 March). The new papers spell out the findings in detail: Near the planet's poles, ice or permafrost lies beneath roughly 30 centimeters of dehydrated rock and soil.

Teams led by William Boynton, a geochemist at the University of Arizona in Tucson, William Feldman, a physicist at Los Alamos National Laboratory in New Mexico, and Igor Mitrofanov, an astrophysicist at Russia's Space Research Institute in Moscow, studied radiation that arises when cosmic rays crash into the red planet. The cosmic rays knock energetic neutrons out of atomic nuclei, and some of these bounce back into space after rattling through the material in the surface and losing energy. Others are recaptured by atomic nuclei, giving off gamma rays whose energy depends on the type of nucleus. The researchers found that the number of intermediate- and low-energy neutrons dipped near Mars's poles, indicating that light, energy-absorbing hydrogen nuclei lurk below the surface. In the same regions, they also saw an increase in gamma rays produced when a hydrogen atom captures a neutron. Because the most likely source of hydrogen in these regions is ice, the two pieces of evidence indicate that below a waterless layer, the martian soil in these regions consists of roughly 35% water by weight, says Feldman.

However, it's too early to say whether such ice can account for much of Mars's missing water, says Jim Bell, a planetary scientist at Cornell University in Ithaca, New York. The neutrons and gamma rays reveal only ice contained in the top meter of material, he says, and that amounts to only a tiny fraction of the water found on the surface of Earth. The real question, Bell says, is how far down does the ice reach? "Is what we're seeing all there is, or are we talking about literally an ocean of water buried underground?"